1. Field of the Invention
The present invention relates to a cross coil type indicating instrument. More particularly, the invention relates to a cross coil type indicating instrument provided with a zero point stopper adjustable in position.
2. Description of the Related Art
There are conventional indicating instruments for automobiles in which an electric current changed depending upon a measured value is supplied to a pair of coils to generate magnetic fields perpendicular to each other and in which a magnetic rotor is rotated in a direction of a combined magnetic field obtained from the magnetic fields generated by the paired coils, whereby indicating the measured value with a pointer rotating with the magnet rotor.
FIG. 3 shows an instrument unit 1 such as a speedometer or a tachometer used in an indicating instrument of such a conventional type. A movement 2 of the instrument unit 1 has a coil bobbin 3, and two coils 4, 4 are wound around the coil bobbin 3 as being perpendicular to each other. A magnet rotor 5 of disk is disposed inside the coil bobbin 3 in the coils 4 with the S pole and the N pole being formed thereon, and a rotation shaft 6 is attached to a central portion of the magnet rotor 5, extending in an axial direction of the coil bobbin 3. The rotation shaft 6 and thus the magnet rotor 5 are freely rotatable with no current through the coils 4. As an electric current flows through the coils 4, the rotation shaft 6 is driven to rotate by a predetermined angle.
A dial plate 7 is attached to the upper face of the movement 2. The fore end of the rotation shaft 6 passes through the dial plate 7 when the movement 2 is mounted on the back face of the dial plate 7. A pointer 8 is fixed at the fore end of the rotation shaft 6.
There is zero return means provided in the movement 2, such as a hair spring (not shown) connected to the coil bobbin 3 at one end, or as a magnet for zero return (not shown). A biasing force of the hair spring or a magnetic force of the magnet for zero return returns the rotation shaft 6 to a predetermined zero position in the state of free rotation of the rotation shaft 6 without an electric current flow through the coils. A movement case 9 surrounds outside the coil bobbin 3 to shield the magnetic field. The instrument unit 1 is thus formed as explained.
In a conventional arrangement, a stopper pin (not shown) stands on the dial plate 7 as means to stop the pointer 8 fixed on the rotation shaft 6 at the zero point on the dial plate 7. The stopper pin stops the pointer 8 at the zero point.
Such an arrangement of the stopper pin on the dial plate 7 as described might give a feeling of complexity to a driver in view of a design on the dial plate. Also the arrangement inconveniently increases the number of components and of assembling steps.
Inventors have proposed a cross coil type indicating instrument as shown in FIGS. 4 and 5, which can properly return the pointer 8 to the zero point on the dial plate 7 without such a stopper pin on the dial plate 7 (Japanese Utility Model Application No Heisei 2-82766).
In the cross coil type indicating instrument proposed, a stopper plate 10 of annular thin plate is disposed between an upper bobbin 3a and a lower bobbin 3b of a coil bobbin 3, and a stop 11 is formed to project downward in FIG. 4 from an inner periphery of a stopper plate 10 to engage with an engagement plate 13 of an annular recess 12 formed on the upper face of a magnet rotor 5, whereby a pointer 8 fixed on a rotation shaft 6 of the magnet rotor 5 may be stopped at the zero point on the dial plate 7.
A protrusion 15 is formed on the outer periphery of the stopper plate 10 with teeth 15a entering a jig hole 14 formed in the outer periphery of the coil bobbin 3. The teeth 15a mesh with a jig (not shown) inserted into the jig hole 14, and the stopper plate 10 may be slightly moved along matching faces between the upper bobbin 3a and the lower bobbin 3b with a rotation of the jig. As the stopper plate 10 moves with use of the jig, the pointer 8 may be adjusted to return to the zero point on the dial plate 7 after completion of assembling of the instrument unit 1.
In the figures, numeral 16 denotes spring pieces, which are formed by upward bending at an inner periphery of the stopper plate 10. The spring pieces 16 contact with an inner face 3a.sub.1 of the upper bobbin 3a, and thereby a lower face of the stopper plate 10 may contact with an upper edge 3b.sub.1 of the lower bobbin 3b with a biasing force of the spring pieces 16. By this arrangement, a torque is maintained constant to hold the stopper plate 10 sandwiched through the spring pieces 16 between the upper bobbin 3a and the lower bobbin 3b, so that a stable rotation torque may be provided in positional adjustment of the stopper plate 10 with the jig.
The above conventional indicating instrument has the following problems.
In the conventional indicating instrument, the spring pieces 16 bent upward at the inner periphery of the stopper plate 10 contact with the upper bobbin 3a, whereby the stopper plate 10 is sandwiched between the upper bobbin 3a and the lower bobbin 3b. In this arrangement, dimensions of the coil bobbin 3 itself of resin mold change with a stress upon a thermal expansion or upon a thermal contraction. This change in dimensions might cause a winding slack in the coils 4, 4 wound around the coil bobbin 3, which would result in reduction in compressive force of the wound coils 4, 4 acting on the coil bobbin 3. Since the stopper plate 10 is held as sandwiched between the upper bobbin 3a and the lower bobbin 3b with the compressive force, the holding torque of the stopper plate 10 will decrease with the reduction in compressive force. This results in movement of the stopper plate 10 already adjusted in position at a predetermined zero point, thereby to in turn move the zero point of the pointer 8. Further, the rotation torque of the stopper plate 10 in the position adjustment thereof with the jig becomes unstable.